1. Field of the Invention
The present invention relates to an objective lens unit capable of making a lens holder of an optical pickup of a disk player smoothly conduct a focusing motion.
2. Description of the Related Art
An example of the conventional objective lens unit is shown in FIGS. 18 to 22. This objective lens unit is composed as follows. At the center of a lens holder 1 to support an objective lens OL, there is provided a base section 1a that is protruded integrally with the lens holder. A shaft hole 2 of the base section 1a is engaged with a shaft 4 arranged on an actuator base 3. In the periphery of the base section 1a , there is provided a focusing coil 5. Further, on both sides of the lens holder 1, there are provided tracking coils 6. Rising plates 3aare raised from both side sections of the actuator base, and the lens holder 1 is interposed between the rising plates 3a. A pair of magnets 7 are arranged on the rising plates 3a. Both end portions of yokes 8, which are screwed to the actuator base 3, are inserted into through-holes 1b of the lens holder 1. A cover 9 is set on the lens holder 1. When a plurality of hooks 9aprotruding from the inner face of the cover 9 are detachably engaged with the actuator base 3, a ceiling section of the cover 9 is contacted with or made to come close to a top face of the shaft 4. In this connection, reference numeral 10 is a printed board, which is connected to the focusing coil 5 and the tracking coil 6 via lead wires 10a. 
In the above constitution, when the focusing coil 5 is energized with electric current so that the focusing coil 5 can be excited, the lens holder 1 is slid in the focusing directions “a ” and “b” along the shaft 4, so that the objective lens OL can be focused on a disk (not shown). When the tracking coil 6 is energized with electric current, the lens holder 1 is oscillated in the tracking directions “c” and “d” around the shaft 4, so that objective lens OL can follow a predetermined track.
In an initial operation test conducted in a factory before the product is shipped, when a predetermined intensity of voltage is impressed upon the focusing coil 5 in a clean room, as shown in FIG. 21, the lens holder 1 is operated in the focusing directions “a” and “b” along the shaft 4 within an allowable range L. At that time, the stroke of the lens holder 1 is checked with a range finder (not shown) such as a laser beam displacement gauge. In the case where the stroke is not proportional to voltage, the product is determined to be defective.
In the above-described constitution, the following problems may be encountered. In the case of a focusing motion of the lens holder 1 conducted at the maximum stroke, an actuator base opposing face 1A formed on an end face of the base section 1a of the lens holder 1 collides with the actuator base 3, and a cover opposing face 1B of the lens holder collides with the cover 9 as shown by the virtual lines in FIG. 21. Therefore, even in a clean room, a very small quantity of dust D attaching to the actuator base 3 and the cover 9 gets into gap α formed between the shaft 4 and the shaft hole 2. Due to dust D which has gotten into gap α in this way, the focusing motion of the lens holder 1 can not be stabilized, and the objective lens unit might be judged to be a defective product.
According to JP-A-11-161984, it is possible to consider the structure shown in FIG. 22 in which bearing contact sections 11, the section of which is semicircular, are protruded at the upper and lower portions on the inner face of a shaft hole 2 so that annular recess sections 12 can be formed at the upper and lower end portions of the shaft hole 2. When a lens holder 1 conducts a focusing motion and comes into contact with an actuator base 3 or a cover 9, each annular recess portion 12 is tightly closed, so that dust D, which has gotten into each annular recess portion 12, can not go anywhere. Accordingly, there is a possibility that dust D in each annular recess portion is pushed into the shaft hole 2. Due to the foregoing, the focusing motion of the lens holder 1 becomes unstable, which might be judged to be a defective product.